Devices and methods for protection of rechargeable elements

ABSTRACT

A protection circuit for use with a charger and a chargeable element, such as a rechargeable lithium ion battery, comprises a shunt regulator having a threshold ON voltage coupled in parallel across the chargeable element, and a temperature-dependent resistor, e.g., a positive temperature coefficient device, coupled in series between the charger and the chargeable element. The temperature dependent resistor is thermally and electrically coupled to the shunt regulator, wherein the first variable resistor limits current flowing through the shunt regulator if the current reaches a predetermined level less than that which would cause failure of the regulator, due to ohmic heating of the regulator.

RELATED APPLICATION DATA

This is a continuation application filed Feb. 27, 2001, of applicationSer. No. 09/425,519, U.S. Pat. No. 6,331,763 filed on Oct. 22, 1999,which is a Continuation-in-part of application Ser. No. 09/060,863,filed on Apr. 15, 1998, abandoned and a continuation-in-part ofprovisional application Ser. No. 60/126,952, filed on Mar. 25, 1999.

FIELD OF INVENTION

The present inventions pertain generally to the field of overvoltage andovercurrent protection systems and more specifically to devices andmethods for protecting rechargeable elements, such as rechargeablebatteries, from overvoltage or overcurrent conditions.

BACKGROUND

Electrical circuits that protect rechargeable elements, such asrechargeable battery packs, are well known. However, such rechargeableelements, and in particular rechargeable lithium battery cells, can bedangerous if the operating voltage exceeds a safe limit.

For example, FIG. 1 shows a typical charging curve, i.e., the voltageacross the battery vs. time, for a common lithium battery pack (e.g.,used for a wireless telephone handset) allowed to keep charging beyondits maximum safe level. As labeled in FIG. 1, this curve may be dividedinto three general areas.

The first area is represented by the region where the voltage, V, isless than 4.5 volts. In this area, the battery charges at a safe level,with the temperature of the battery remaining below 60° C. to 70° C.,and the pressure inside the battery remaining below 3 bars.

The second area is represented by the region where the voltage isbetween 4.5 volts and 5.3 volts. When charging is in this area, thebattery begins to operate in a dangerous mode, with the temperaturerising above 70° C., and the pressure inside the battery rising to arange between 3 bars to 10 bars. Even at this slightly increased voltagelevel, the battery might even explode.

The third area is represented by the region where the voltage exceeds5.3 volts. At this stage, it is too late to save the battery, which issubjected to internal degradation and may explode or combust. Notably,battery cells in a “fully-charged” state are more dangerous andsusceptible to explosion than those in the discharged state.

In particular, in order to be sure that a lithium battery operates inits safe operating mode during a charging operation, at least one of thefollowing three conditions must be met: 1) temperature<60° C., 2)pressure<3 bars, or 3) voltage<4.5 volts.

Towards this end, rechargeable lithium ion battery packs areconventionally provided with a “smart” electronic circuit in series withthe batteries to provide protection against exposure to an excessivevoltage or current. Such smart protection circuits may also guardagainst an undervoltage condition caused by overdischarge of the batterypack.

By way of example, a conventional “smart” protection circuit 21 for arechargeable lithium ion battery pack is shown in FIG. 2. In particular,first and second MOSFET switches 20 and 22 are placed in series with oneor more battery cells 24. The MOSFET switches 20 and 22 are switched ONor OFF by control circuitry 26, which monitors the voltage and currentacross the battery cell(s) 24. In normal operation, the MOSFET switches20 and 22 are switched “ON” by the control circuitry 26 to allow currentto pass through in either direction for charging or discharging of thebattery cell(s) 24. However, if either the voltage or current across thebattery cell(s) 24 exceeds a respective threshold level, the controlcircuitry 26 switches OFF the MOSFETs 20 and 22, thereby opening thecircuit 21. The control circuitry 26 also monitors the voltage andcurrent levels across a charging source 28 to determine when it is safeto switch back ON the respective MOSFETs 20 and 22.

As will be appreciated by those skilled in the art, the smart protectioncircuit 21 is relatively complex and expensive to implement with respectto the overall expense of a conventional battery pack Further, theseries resistance across the MOSFETs 20 and 22 is relatively high,thereby decreasing the efficiency of both the charging source 28 and thebattery cells 24. Notably, both MOSFETs 20 and 22 are needed to preventcurrent from passing in either direction when the circuit is open,—i.e.,by way of respective body diodes 23 and 25 biased in oppositedirections—, which increases the complexity, cost and total in-seriesresistance of the protection circuit 21. Also, because the MOSFETS 20and 22 are subject to failure if exposed to a sudden high voltage (oruse of an improper high voltage charger), secondary protection of thebattery cell(s) 24 is still needed, such as, e.g., a positivetemperature coefficient (“PTC”) resettable fuse employed in series witheach cell.

By way of background information, devices exhibiting a positivetemperature coefficient of resistance effect are well known and may bebased on ceramic materials, e.g., barium titanate, or conductive polymercompositions. Such conductive polymer compositions comprise a polymericcomponent and, dispersed therein, a particulate conductive filler. Atlow temperatures, the composition has a relatively low resistivity.However, when the composition is exposed to a high temperature due, forexample, to ohmic heating from a high current condition, the resistivityof the composition increases, or “switches,” often by several orders ofmagnitude. The temperature at which this transition from low resistivityto high resistivity occurs is called the switching temperature, Ts. Whenthe device cools back below its switching temperature Ts, it returns toa low resistivity state. Thus, when used as an in-series currentlimiter, a PTC device is referred to as being “resettable,” in that it“trips” to high resistivity when heated to its switching temperature ,Ts, thereby decreasing current flow through the circuit, and thenautomatically “resets” to low resistivity when it cools back below Ts,thereby restoring full current flow through the circuit after anovercurrent condition has subsided.

In this application, the term “PTC” is used to mean a composition whichhas an R14 value of at least 2.5 and/or an R100 value of at least 10,and it is preferred that the composition should have an R30 value of atleast 6, where R14 is the ratio of the resistivities at the end and thebeginning of a 14° C. range, R100 is the ratio of the resistivities atthe end and the beginning of a 100° C. range, and R30 is the ratio ofthe resistivities at the end and the beginning of a 30° C. range.Generally the compositions used in devices of the present inventionsshow increases in resistivity, which are much greater than those minimumvalues.

Suitable conductive polymer compositions are disclosed in U.S. Pat. Nos.4,237,441 (van Konynenburg et al), 4,545,926 (Fouts et al), 4,724,417(Au et al), 4,774,024(Deep et al), 4,935,156 (van Konynenburg et al),5,049,850 (Evans et al), 5,250,228 (Baigrie et al), 5,378,407 (Chandleret al), 5,451,919 (Chu et al), 5,582,770 (Chu et al), 5,701,285(Chandler et al), and 5,747,147 (Wartenberg et al), and in co-pendingU.S. application Ser. No. 08/798,887 (Toth et al, filed Feb. 10, 1997).The disclosure of each of these patents and applications is incorporatedherein by reference for all that it discloses.

Referring to FIG. 3A, a crowbar type protection circuit 31 is also wellknown. In particular, a switch element 30 is placed in parallel acrossthe battery cell(s) 24. The switch 30 is opened or closed by controlcircuitry 36, which monitors the voltage and current across the batterycell(s) 24. In normal operation, the switch 30 is left open. However, ifeither the voltage or current across the battery cell(s) 24 exceeds arespective threshold, the control circuitry 36 closes the switch 30,thereby shorting the circuit across the battery cell(s) 24.

FIG.3B illustrates the current versus voltage curve 35 through theswitch element 30, when it is closed. Notably, the current can quicklyreach relatively high levels, depending on the characteristics andduration of a particular power surge. Towards this end, a firstovercurrent element 32 may be provided between the switch element 30 andthe charging element 28 to help protect the switch element 30 fromcontinuous current from the charging element 28. Similarly, a secondovercurrent element 34 may be provided between the switch element 30 andthe battery cell(s) 24, in order to protect the cell(s) 24. However, thecombined in-series resistance of the overcurrent elements 32 and 34 isundesirable across the battery path.

FIG. 4A depicts an alternate overvoltage protection clamping circuit 41.In particular, a voltage clamping element 40, such as a zener diode, isused in place of the switch element 30 in parallel with the batterycell(s) 24. In an overvoltage condition, the clamping element 40 limitsthe voltage across the battery cell(s) 24.

FIG. 4B illustrates the current versus voltage curve 45 for the clampingcircuit 41. As with the crowbar circuit 31, current through the clamp 40can quickly reach relatively high levels, depending on thecharacteristics and duration of a particular voltage spike. Again,placement of current limiting elements (not shown in FIG. 4) can protectthe clamp 40 and/or battery cell(s) 24 from excessive current. Notably,the clamping element 40 can have a relatively high current leakage,e.g., as in the case of a zener diode, causing the battery cell(s) 24 tolose their charge quickly over time.

SUMMARY OF THE INVENTIONS

In accordance with one aspect of the inventions disclosed herein, aprotection circuit for use with a charger and a chargeable elementincludes a shunt regulator having first and second terminals forcoupling in parallel across the chargeable element, the shunt regulatorhaving a threshold ON voltage. A temperature dependent resistor isthermally and electrically coupled to the shunt regulator, the resistorhaving a first terminal for coupling to the charger in series and asecond terminal for coupling to the chargeable element in series.

In a preferred embodiment, the resistor is a positive temperaturecoefficient device, which switches from a relatively low resistance to arelatively high resistance when heated to a certain transitiontemperature. The transition temperature of the device is selected suchthat current flowing though the shunt regulator in an overvoltagecondition causes sufficient ohmic heat generation in the shunt regulatorto heat the device to the transition temperature prior to failure of theshunt regulator.

In accordance with another aspect of the inventions disclosed herein,control circuitry for controlling a shunt regulator transistor switch ina protection circuit for use with a charger and a chargeable elementincludes first and second voltage detection circuits. The first voltagedetection circuit is relatively low leakage and is configured toactivate the second voltage detection circuit if the voltage across thechargeable element approaches a threshold ON voltage of the transistorswitch. The second voltage detection circuit is relatively precise andis configured to activate the transistor switch if the voltage acrossthe chargeable element reaches the threshold ON voltage.

In accordance with yet another aspect of the inventions disclosedherein, control circuitry for controlling a shunt regulator transistorswitch in a protection circuit for use with a charger and a chargeableelement includes an operational amplifier having an output coupled to anactivation gate of the transistor switch, and a voltage clamping elementcoupled to the operational amplifier output, the clamping elementeffectively clamping the activation gate voltage.

In accordance with still another aspect of the inventions disclosedherein, a shunt regulator for protecting a chargeable element fromovercharging includes a transistor switch having a thermally-compensatedvoltage characteristic.

In accordance with yet another aspect of the inventions disclosedherein, a protection circuit for use with a charger and a chargeableelement includes an overvoltage shunt regulator having first and secondterminals for coupling in parallel across the chargeable element, and anundervoltage protection circuit having first and second configured forcoupling in series between the charger and the chargeable element. In apreferred embodiment, the overvoltage shunt regulator comprises a firsttransistor switch having a threshold ON voltage approximating a selectedmaximum operating voltage of the chargeable element. The undervoltageprotection circuit comprises a second transistor switch having athreshold ON voltage approximating a selected minimum operating voltageof the chargeable element.

In accordance with a still further aspect of the inventions disclosedherein, a protection circuit for use with a charger and a chargeableelement includes an overvoltage shunt regulator having first and secondterminals for coupling in parallel across the chargeable element, theshunt regulator comprising a MOSFET switch having a threshold ON voltageapproximating a selected maximum operating voltage of the chargeableelement, and a relatively high resistance, reverse-current body diode.

In accordance with a still further aspect of the inventions disclosedherein, a protection circuit for use with a charger and a chargeableelement includes an overvoltage shunt regulator having first and secondterminals for coupling in parallel across the chargeable element, theshunt regulator comprising a transistor switch having a channel throughwhich current may flow in a forward direction if positive-biased, or areverse direction if negative-biased. In particular, the transistorswitch is constructed such that current flowing in the forward directionencounters relatively low resistance, and current flowing in the reversedirection encounters relatively high resistance.

In accordance with yet another aspect of the inventions disclosedherein, a protection circuit for use with a charger and a chargeableelement includes a shunt regulator having first and second terminals forcoupling in parallel across the chargeable element, the shunt regulatorhaving a threshold ON voltage. A first positive temperature coefficient(PTC) device is thermally and electrically coupled to the shuntregulator, the first PTC device having a first terminal for coupling tothe charger in series and a second terminal for coupling to thechargeable element in series. A second PTC device is coupled in serieswith the shunt regulator, wherein the transition temperature of thefirst PTC device is lower than that of the second PTC device.

In accordance with yet another aspect of the inventions disclosedherein, a protection circuit is provided in combination with a batteryhaving a positive terminal and a negative terminal, the protectioncircuit including a transistor switch coupled in series with thebattery, such that, when the transistor switch is ON, the battery willdischarge through a load. A temperature-dependent resistor is thermallycoupled to the transistor switch, the temperature-dependent resistorhaving a first terminal coupled to the positive battery terminal. Afixed resistor is provided having a first terminal coupled to a secondterminal of the temperature dependent resistor, and a second terminalcoupled to the negative battery terminal, such that the respectivetemperature dependent resistor and fixed resistor are coupled in serieswith each other and in parallel with the battery. The transistor switchhas an activation gate coupled in a divider configuration to the secondterminal of the temperature dependent resistor and first terminal of thefixed resistor.

In a preferred embodiment, the temperature dependent resistor switchesfrom a relatively low resistance to a relatively high resistance at atransition temperature selected such that, in an overvoltage conditionin the battery, current flowing through the transistor switch willgenerate sufficient ohmic heat to heat the temperature dependentresistor to the transition temperature before casing failure of thetransistor switch.

As will be apparent to those skilled in the art, other and furtheraspects and advantages of the present invention will appear hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present inventions taught herein areillustrated by way of example, and not by way of limitation, in thefigures of the accompanying drawings, in which similar elements in thedifferent embodiments are referred to by the same reference numbers forease in illustration, and in which:

FIG. 1 shows a charging curve of a typical rechargeable lithium battery;

FIG. 2 is a schematic illustration of a “smart” overvoltage protectioncircuit employing a pair of transistor elements in series with arechargeable battery pack;

FIG. 3A is a schematic illustration of a “crowbar” protection circuitemploying a voltage controlled short circuit switch in parallel with arechargeable battery pack;

FIG. 3B depicts the current vs. voltage relationship of the crowbarcircuit of FIG. 3A;

FIG. 4A is a schematic illustration of a voltage clamping circuitemploying a zener diode voltage clamp in parallel with a rechargeablebattery pack;

FIG. 4B depicts the current vs. voltage relationship of the clampingcircuit of FIG. 4A;

FIGS. 5-10 are simplified block diagrams and circuit schematics ofpreferred secondary protection circuits, according to one aspect of thepresent inventions;

FIG. 11 is a simplified block diagram of a further preferredovervoltage, overcurrent primary protection circuit in accordance withanother aspect of the present inventions, including a PTC device inseries, and a voltage regulator in parallel, respectively, with arechargeable battery pack;

FIG. 12 is a schematic illustration of the protection circuit of FIG.11, with the PTC device thermally coupled to the regulator;

FIG. 13 is a schematic illustration of a preferred variation of theprotection circuit of FIGS. 11 and 12;

FIGS. 14-16 depict preferred current-voltage relationships forprotection circuits in accordance with a further aspect of the presentinventions;

FIG. 17 depicts a preferred thermally-compensated voltage characteristicfor a preferred protection circuit;

FIG. 18 is a thermal model circuit representation for the protectioncircuit of FIGS. 12-13;

FIG. 19 depicts the power dissipated through the respective regulatorand PTC device in the circuit of FIGS. 12-13 during an overvoltagecondition;

FIG. 20 is a schematic illustration of a preferred overvoltage,overcurrent protection circuit employed with a rechargeable batterypack;

FIG. 21 is an alternate preferred embodiment of the protection circuitof FIG. 20;

FIG. 22 is a simplified block diagram of the circuit of FIG. 20;

FIG. 23 is a simplified block diagram of a further alternate preferredembodiment of the circuit of FIG. 20, employing a low leakage activationcircuit;

FIG. 24 is a schematic of a further preferred protection circuit,including both over and undervoltage protection circuits, in accordancewith yet another aspect of the present inventions;

FIG. 25 is a schematic showing the body diode of the regulator MOSFETelement in FIG. 20;

FIG. 26 is a schematic showing the addition of a resistance in serieswith the body diode in the MOSFET of FIG. 25;

FIG. 27 is a cross-sectional view of a preferred MOSFET device for useas the shunt regulator with the added series resistance in theprotection circuit of FIG. 26;

FIG. 28 illustrates an alternate preferred semiconductor device for usein preventing reverse battery discharge, in accordance with a stillfurther aspect of the present inventions;

FIG. 29 illustrates a further alternate preferred semiconductor devicefor use in preventing reverse battery discharge;

FIG. 30 depicts a preferred current-voltage curve for the devices ofFIGS. 28 and 29;

FIG. 31 is a simplified schematic of a further preferred overvoltageprotection circuit;

FIGS. 32-33 depict the current-voltage relationship of the circuit ofFIG. 31;

FIG. 34 is a simplified schematic of a still further preferredovervoltage protection circuit;

FIG. 35 depicts the current-voltage relationship of the circuit of FIG.34;

FIG. 36 is a simplified schematic of a yet another preferred overvoltageprotection circuit;

FIGS. 37-38 depict the current-voltage relationship of the circuit ofFIG. 36;

FIG. 39 is a simplified schematic of a preferred three terminalprotection circuit;

FIGS. 40-42 are simplified schematic diagrams of alternate embodimentsof the three terminal protection device of FIG. 39;

FIG. 43 is a simplified schematic diagram of a prior art overdischargeprotection circuit for preventing overdischarge of a battery;

FIG. 44 is a simplified schematic diagram of a preferred overdischargeprotection circuit, in accordance with yet another aspect of the presentinventions;

FIG. 45 is a side view of a preferred three terminal protection device,including a MOSFET regulator thermally and electrically coupled to a PTCchip, partially cut-away to show an internal portion of the regulator;-

FIGS. 46-47 are respective top and bottom side perspective views of thedevice of FIG. 45;

FIG. 48 is a perspective view of a sheet of PTC material sectioned forcutting into a plurality of PTC devices during assembly of the device ofFIG. 45;

FIG. 49 is a perspective view of a preferred lead frame for use in themanufacture of injection molded housings for the three terminal deviceof FIG. 45;

FIG. 50 is a perspective view of a plurality of molded housings formedon the lead frame of FIG. 49;

FIG. 51 is a perspective view of the three terminal device of FIG. 45seated in a housing formed in accordance with the process depicted inFIGS. 49-50, without a cover;

FIG. 52 is a perspective view of the three terminal device of FIG. 45seated in a housing formed in accordance with the process depicted inFIGS. 49-50, with a cover;

FIG. 53 is a perspective view of a first alternate preferred lead framefor use in the manufacture of injection molded housings for the threeterminal device of device of FIG. 45;

FIG. 54 is a perspective view of a plurality of molded housings formedon the lead frame of FIG. 53;

FIG. 55 is a perspective view of a second alternate preferred lead framefor use in the manufacture of injection molded housings for the threeterminal device of device of FIG. 45;

FIG. 56 is a perspective view of the three terminal device of FIG. 45seated in a housing formed in accordance with the process depicted inFIGS. 54 or 55;

FIG. 57 is a side view of an alternate embodiment of the preferred threeterminal protection device of FIG. 45;

FIG. 58 is a bottom side perspective view of the device of FIG. 57;

FIG. 59 is an elevated perspective view of a portion of a flexibleprinted circuit board (“pc board”) configured with an opening formounting an alternate preferred three terminal protection circuit to arechargeable battery pack;

FIG. 60 is an elevated perspective of the flexible pc board of FIG. 59,depicting a MOSFET regulator device mounted, through the opening, to aPTC device secured to an underlying side of the pc board;

FIG. 61 is an elevated perspective of the underlying side of the pcboard of FIG. 59; and

FIG. 62 is a partially cut-away side view of the pc board of FIG. 61.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with a first aspect of the present inventions disclosedherein, a protection system is provided which protects, during acharging operation, a rechargeable, battery against being overchargedinto a dangerous operating mode. The protection system of theembodiments of FIGS. 5-10 may be used as a back up system and willgenerally be used in conjunction with a smart power circuit thatmonitors the charge of the battery. Typically, this protection systemcould be incorporated into the battery itself, or it could be used aspart of the pack electronics, or as part of the charger.

FIGS. 5-7 show the basic circuit embodiments according to this aspect ofthe present inventions. In FIG. 5, a rechargeable battery 1, such as,e.g., a lithium battery with a maximum operating voltage of 4.5 volts iscoupled, in parallel, to a voltage-dependent resistive element, such asa 4.1 volt zener diode 2, forming a parallel circuit. The parallelcircuit is coupled in series with a protection element 3, such as a PTCdevice, a thermal fuse or a bimetallic breaker.

Protection element 3 is preferably thermally coupled to zener diode 2 inorder to accelerate the activation of the protection element 3. Aparallel circuit of a charger 5 and a smart circuit 6 are connected inseries with the parallel combination of battery 1 and zener diode 2.Charger 5 is also connected to a power source (not shown). In thisembodiment, since protection element 3 is connected in series with theparallel circuit of battery 1 and zener diode 2, the total amount offault current flows in protection element 3 and therefore the protectionelement will be activated faster.

FIGS. 6 and 7 show variations of the embodiment of FIG. 5, including useof an additional protection element 4.

In the above embodiments, since power dissipation in the zener diode 2is a large value between 1 to 4 watts, the power dissipation can causean efficient, thermally-assisted tripping of the protection element 3.The protection element 3 and the zener diode 2 may be hybridized toimprove the thermal coupling.

When a constant current DC charger 5 (which is generally the case) isused, the current begins to charge battery 1 because of its low internalresistance. If the smart circuit fails to operate, as soon as thebattery voltage reaches 4.3 volts, a small current is diverted intozener diode 2, which maintains the voltage at 4.3 volts. If the chargecurrent becomes higher, the differential resistance of the zener diode 2will decrease by accepting more and more current to maintain a 4.3 voltconstant voltage over the battery 1. In this case, the zener diode 2 isin runaway mode and the zener diode is heated up. The heat dissipated bythe zener diode makes the protection element trip faster, thus avoidingovercharging the battery 1 into a dangerous operating mode.

When a constant voltage charger is used as power source 5, the circuitsin FIGS. 5-7 operate in a similar manner as the above.

FIG. 8 shows another embodiment of the invention in which anopto-coupler 7 is used. Opto-coupler 7 includes a receiving element,such as a phototransistor 8 and a transmitting element, such as an LED(light-emitting diode) 9. As shown in FIG. 8, rechargeable battery 1 iscoupled, in parallel, to the series combination of zener diode 2 with a3-volt rating, for example, and LED 7 to form a first parallel circuit.A protection element 3, such as a fuse, a PTC device or a bimetallicbreaker, is coupled in series with the first parallel circuit.Phototransistor 8 is coupled in parallel with the combination ofprotection element 3 and the first parallel circuit to form a secondparallel circuit. The parallel circuit of a charger 5 and a smartcircuit 6 is coupled in parallel with the second parallel circuit.Charger 5 is also connected to a power source (not shown).

The embodiment of FIG. 8 operates according to principles similar tothose described above. Under normal conditions, the current in the zenerdiode 2 is not sufficient to light LED 9. However, if a fault occurs,e.g., a high voltage charger is used, the current in zener diode 2 willincrease and thus activate opto-coupler 7, which in turn shunts battery1. This causes protection element 3 to activate to thereby disconnectbattery 1.

FIG. 9 shows yet another preferred protection circuit. In thisembodiment, rechargeable battery 1 is coupled in parallel with anovercharge detection device 10, such as an overvoltage detection device,Model No. TC54VN, (e.g., packages SOT 23B-3 or SOT89-3), manufactured byTelcom Semiconductor, Inc. The parallel circuit of battery 1 anddetection device 10 is coupled in series with a protection element 11,which may be a fuse, a thermal fuse or a PTC device. The combination ofbattery 1, detection device 10 and protection element 11 is coupled inparallel with a power MOSFET transistor 12, such as Motorola MTD 3055EL(VL), case 369A-10. The parallel circuit of a smart circuit 6 and acharger 5 is coupled to battery 1 in parallel. Charger 5 is alsoconnected to a power source (not shown).

MOSFET 12 is biased by a resistor 13 and driven by detection device 10via transistor 14. Detection device 10 includes a constant currentgenerator 15, which supplies current to a reference zener diode 16. Thevoltage of zener diode 16 is compared with the battery voltage using anoperational amplifier 17, connected to resistors 18 and 19, as acomparator. When the. battery voltage reaches 4.5 volts, comparator 17outputs a positive voltage which turns off transistor 14, which thenturns on MOSFET 12. This causes battery 1 to be shunted. Thus, a highcurrent flows in protection element 11. If a fuse (such as theAVX-Kyocera by Farnell) is used as protection element 11, it will blowand disconnect battery 1, thus preventing the battery from exploding. Ifa PTC device is used as protection element 11 instead of a fuse, the PTCwill trip and reduce the high current to a low leakage current, thuspreventing the battery from exploding.

Under normal conditions, when smart circuit 6 functions properly, thebattery voltage is below the voltage of zener diode 16. Thus, comparator17 outputs a negative voltage which turns on transistor 14, which causespower MOSFET 12 to be in its off state.

In the embodiment of FIG. 9, a constant current charger may be usedwithout danger. Assuming the maximum charge current of battery 1 is 2Cwhere C is the battery capacity specified by the manufacturer, if thecharge current exceeds 2C, the fuse will blow and disconnect battery 1from charger 5. However, if the charge current is within 2C but thecharge voltage is higher than 4.5 volts, detection device 10 will detectthe fault and shunt battery 1, causing the fuse to blow.

A constant voltage charger may also be used without danger in theembodiment of FIG. 9.

If the voltage of charger 5 is too high and a charge current higher than2C is induced, the fuse will blow and prevent the battery fromexploding. On the other hand, if the charge current is less 2C but thevoltage across the battery is greater than 4.5 volts because of the highvoltage of charger 5, then overvoltage detection device 10 will play itsrole by shunting the battery and blowing the fuse. If a PTC device isused in place of the fuse, the PTC device will trip, thus protecting thebattery.

In the embodiment of FIG. 9, all the components (i.e., the protectionelement, overcharge detection device, resistor and power MOSFET) arepreferably surface mounted devices (SMD).

Referring to FIG. 10, in a still further preferred embodiment, arechargeable battery 1, such as a lithium ion battery, is coupled inparallel with an alternate overcharge detection device 27, such as ModelNo. TC54VC, by Telcom Semiconductor, Inc. The detection device 27includes a constant current generator 15, a zener diode 16, resistors 18and 19, operational amplifier 17, a p-type field effect transistor (FET)Q1, and an n-type FET Q2. The parallel circuit of battery 1 anddetection device 27 is coupled in series with a first protection element99. An output of detection device 27 provides control over a thyristor(SCR) 43 via a resistor R1. A second protection element 98 is connectedin series with the parallel circuit of the first protection element 99,thyristor 43, detection device 27 and battery 1. A charger 5 is to beconnected to the overall circuit. The charger 5 is also connected to apower source (not shown). In a preferred embodiment, each of the twoprotection elements 99 and 98 may be a fuse with a delay feature, suchas a SMD Slo-Blo fuse 2A, commercially available from Littelfuse.

Such a fuse typically has a delay of approximately twenty ms uponoccurrence of a high current before it blows. If the high currentdisappears within this time duration, the fuse will not blow. Also, R1may be a SMD resistor with a resistance value of 22 kΩ. An example ofthyristor 43 may be a ST 1220-600B thyristor, commercially availablefrom ST Microelectronics (France). Under normal conditions, the charger5 provides a regulated voltage of 4.3 V and supplies a current of twoamps via protection elements 99 and 98 to battery 1. A detected voltageVd is compared with a reference voltage, Vref, using the operationalamplifier 17 as a comparator. In this case, the detected voltage Vd isbelow the reference voltage Vref. Thus, comparator 17 outputs a positivevoltage, which will turn on transistor Q2, while transistor Q1 remainsoff. Since there is no current flowing through resistor R1, thethyristor 43 is not activated, and a normal charging operation isperformed.

In the case in which a wrong charger is used, i.e., a charger with ahigh voltage rating, such as, e.g., a 12 V charger, the battery voltageVbat will exceed 4.3 V after Vd exceeds Vref. In such case, thecomparator 17 outputs a negative voltage, which turns on transistor Q1,while transistor Q2 is off. This causes a current to flow throughresistor R1 and to the gate of thyristor 43. Thus, the thyristor 43 isactivated and shorts the battery 1 and the charger 5. As a result, ahigh current is drawn from the battery 1 and the charger 5, throughthyristor 43, to ground. The high current causes protection elements 99and 98 to blow, thus disconnecting the (wrong) charger 5 from thebattery 1. The delay feature of protection elements 99 and 98advantageously prevents accidental shorting of the battery that lastsfor only a very short period of time.

Notably, preferred embodiments of further inventions and inventiveaspects disclosed and described herein are directed primarily to standalone protection or regulator circuits,—i.e., and are not intended assecondary back-up to a smart circuit, as was the case with the abovedescribed preferred embodiments of FIGS. 5-10.

For ease in illustration of further inventions and inventive aspectsdisclosed and described herein, the basic elements of a preferredovervoltage, overcurrent protection circuit 37 are depicted in FIG. 11.In particular, a voltage regulator (e.g., a voltage controlled MOSFETswitch) 39 is placed in parallel with the battery cell(s) 24. A PTCdevice 38 is provided between the regulator 39 and the charging element28, wherein the PTC device 38 is in series with the battery cell(s) 24.

Referring to FIG. 12, the regulator 39 is preferably thermally coupledto the PTC device 38, as indicated by arrow 48. In an overvoltagecondition, the regulator 39 limits the voltage across the batterycell(s) 24, causing power to be dissipated in the form of currentpassing through the regulator 39. This current generates heat in theregulator, which is conducted to the PTC device 38, increasing thetemperature of the PTC device to its switching or “trip” temperature. Atthat point the PTC device 38 rapidly increases in resistance, whichcorrespondingly substantially decreases the current passing through theregulator 39, with the thermal equilibrium of the circuit 37 determiningthe ultimate operating point, until the overvoltage condition is over.At that time, the regulator 39 stops conducting current, and the PTCdevice cools back below its trip temperature, thereby restoring thecircuit 37 to its normal operating condition.

As will be apparent to those skilled in the art, the PTC device 38 mayequally be deployed in the ground path of the battery charging circuit,as shown in the alternate protection circuit embodiment 37′ of FIG. 13.As is explained in greater detail herein, a design choice betweenembodiments 37 and 37′ will hinge on how the thermal link 48 between thePTC device 38 and regulator 39 is physically manifested.

In either embodiment 37 or 37′, the PTC device 38 also serves to protectagainst an overcurrent caused by a sudden charging or discharging of thebattery cell(s) 24. In particular, should there be a sudden rise in thecurrent, the PTC device 38 will experience rapid ohmic heating from thesudden surge in dissipated power, until it trips and substantiallychokes back on the current.

Importantly, in order to provide for efficient charging and dischargingof the battery cell(s)24, the in-series resistance of the PTC device 38and leakage of the regulator device 39 are preferably minimized.

Notably, the shunt regulator 39 of protection circuit embodiments 37 and37′ is expected to experience a current-voltage relationship representedby curve 45 in FIG. 4B. However, if a large surge voltage with lowsource resistance is applied across charger or cell terminal(s) 24, theshunt regulator 39 will be overloaded and quite possibly destroyed.

Towards this end, prior art circuits are typically characterized by lowsurge rating capabilities for the silicon shunt regulator. However, inaccordance with a further aspect of the present inventions, a shuntregulator can be configured to have specific advantageouscurrent-voltage relationships, so as to limit the power dissipationrequirements of the regulator—i.e., so as to optimize the regulator forbattery protection circuits.

Three alternate preferred current-voltage characteristics for aprotection circuit shunt regulator are presented in FIGS. 14-16.

In FIG. 14, the I-V curve 421 achieves a plateau 422 when the current Ireaches a selected maximum current level I_lim. In other words, theregulator device is designed to withstand a given overload, so long asthe maximum current limit for the particular design, I_lim, is notexceeded. In particular, by limiting the maximum currents andcorresponding voltages for a given regulator design, the power requiredto be dissipated can thereby be limited.

A more complex circuit can be used to further reduce the dissipation byintroducing a second current limit, triggered at a preset voltage, Vt.Towards this end, in FIG. 15, the initial portion of the curve 423 issimilar to curve 421. A plateau 424, similar to plateau 422 is reachedwhen the current reaches I_lim1. However, as the voltage level increasesat constant I_lim1, thereby increasing the power dissipationrequirements of the shunt regulator, the current level steps down at atrigger voltage Vt. When the voltage level reaches preset Vt, thecurrent I drops from I_lim1 to a lower current level I_lim2 (425). Thisdrop in current advantageously reduces power dissipation requirements ofthe shunt regulator.

FIG. 16 shows yet another embodiment of a preferred I-V relationshipthat reduces the power dissipation requirements for shunt regulators atrelatively high currents. The initial slope 426 is like curves 421 and423, prior to reaching plateaus. However, when the voltage referencereaches a value corresponding to I_max, the shunt regulator elementlatches to a low-voltage/high-current mode, represented by plateau 427.At Imax, the I-V characteristic of the shunt regulator are engineered tosupport a voltage drop to Von at Imax. At this reduced voltage level,the shunt regulator can handle higher current levels, as represented bycurve 428.

Referring to FIG. 17 with reference still to the protection circuitembodiments 37 and 37′ of FIGS. 12-13, the switching voltage of theregulator 39 can also be implemented as a function of temperature. Inparticular, it may be desirable to implement a thermally-compensatedvoltage characteristic in the regulator 39, such as that represented bythe temperature-voltage curve 49. In particular, a regulator 39 with aswitching characteristic following the temperature-voltage curve 49allows the switching voltage to be set significantly lower than theexpected use-temperature of the battery 24.

For example, above a certain safe-use temperature, say 80° C.,implementing the temperature- voltage curve 49 would allow the battery24 to discharge through the regulator 39 if the safe-use temperature ofthe battery 24 is exceeded. In other words, the regulator 39 would actas a passive overtemperature protector, as well as a protector forovervoltage and overcurrent conditions.

As will be appreciated by those skilled in the art, many if not all ofthe preferred regulator embodiments disclosed and described herein couldbe designed or implemented to include at least one of the voltagecharacteristics shown in FIGS. 14-17.

FIG. 18 depicts an equivalent thermal circuit model for the protectioncircuit of FIGS. 12-13, which can be represented as an RC circuit.

In particular, for a PTC device (such as the PTC device 38), thetemperature T can be determined from the following equation:$\begin{matrix}{\frac{E}{t} = {{{mCp}\frac{T}{t}} + {k\left( {T - T_{a}} \right)}}} & (1)\end{matrix}$

where $\frac{E}{t}$

is the energy per unit time (power), m is the mass,

C_(p) is the specific heat, k is the thermal resistance and

T_(a) is the ambient temperature

For a parallel RC circuit, the voltage V across the circuit is:$\begin{matrix}{I = {{C\frac{V}{t}} + \frac{V}{R}}} & (2)\end{matrix}$

where I is the current into the circuit C is the capacitance and R isthe resistance. $\begin{matrix}{\frac{E}{t} = {{{mCp}\frac{T}{t}} + {k\left( {T - T_{a}} \right)}}} & (1)\end{matrix}$

where $\frac{E}{t}$

is the energy per unit time (power), m is the mass,

C_(p) is the specific heat, k is the thermal resistance and

T_(a) is the ambient temperature

In comparing equations (1) and (2), dE/dt is analogous to current I, Tis analogous $\begin{matrix}{I = {{C\frac{V}{t}} + \frac{V}{R}}} & (2)\end{matrix}$

where I is the current into the circuit

C is the capacitance and R is the resistance, to voltage V, MC_(p) isanalogous to capacitance C, and k is analogous to conductance 1/R.

Returning to the thermal model in FIG. 18, capacitance 52 represents thethermal capacitance (mC_(p))_(reg) of the regulator device 39,resistance 54 represents the thermal resistance R_(θ(Reg-Ambient)) ofthe regulator-to-ambient heat path, and the power dissipated in theregulator, P_(d(reg)), is represented by current source 46. Inparticular, the thermal capacitance determines how much energy isrequired to increase the temperature of the regulator 39, i.e., thegreater the thermal mass, the greater the energy required to raise thetemperature. The thermal resistance determines how effectively that heatcan be dissipated. A larger thermal resistance will mean that heat isnot dissipated to the surroundings as effectively as it would be with alower thermal resistance.

In a similar fashion, capacitance 60 represents the thermal capacitance(mC_(p)PTC) of the PTC device 38, resistance 58 represents the thermalresistance R_(θ(PTC-Ambient)) of the PTC-to-ambient heat path, and thepower dissipated in the PTC device, P_(d(PTC)), is represented bycurrent source 47.

When power is dissipated in the regulator 39, the temperature, or“voltage” of the thermal capacitance will increase. The thermalresistance Rθ(Reg-Ambient) will conduct heat to the ambient, preventingthe temperature of the regulator 39 from increasing indefinitely. Inthis regard, the lower the thermal resistance to ambient, the lower thetemperature rise of the regulator element.

Likewise, some of the heat will be conducted from the regulator 39 tothe PTC device 38 through the thermal resistance, Rθ(Reg-PTC) 56,between the regulator 39 and the PTC device 38. This thermal link causesa temperature rise in the PTC device as the temperature of the regulator39 increases. Once the PTC device 38 reaches its switching temperature,the PTC device 38 will trip and limit the power in the regulator 38.Thus, to limit the temperature increase of the regulator 39, it isdesirable for the PTC device 38 to reach its switching temperature asquickly as possible. Towards this end, the thermal resistance betweenthe PTC device 38 and regulator 39 should be made as small as possible.

The temperature rise of the regulator 39 can also be limited byemploying a PTC device with a relatively low switching temperature sothat the PTC device 38 will trip relatively quickly in the event theregulator 39 begins heating up in an overvoltage or overcurrentsituation. Also, by reducing the mass of the PTC device 38, its thermalcapacitance 60 is reduced and, as heat is transferred into the PTCdevice 38, its temperature will increase more quickly. As can beobserved from the circuit model of FIG. 18, it is preferable to make thethermal capacitance 60 of the PTC as small as possible. For example,U.S. Pat. No. 5,801,612 issued to Chandler et al, which is fullyincorporated by reference herein for all that it teaches, discloses apreferred low temperature activated, lower mass, PTC material.

Depicted in FIG. 19 is graphical representation of power dissipated inthe protection circuit of FIGS. 12-13 during an overvoltage condition.

Below a certain threshold voltage 63, negligible power is dissipated bythe either the regulator 39 or the PTC device 38,—i.e., the batterycircuit is operating or being charged within a normal operating voltage.Should the voltage rise above a threshold maximum, however, theregulator 39 begins to conduct current and dissipate heat, which isrepresented by curve 64. As the PTC device 38 increases temperature dueto the heat conducted from the regulator 39, it reaches its switchingtemperature and begins to dissipate a greater amount of power,represented by curve 66. As the PTC device 38 dissipates more power,less current passes through the regulator 39, which correspondinglydissipates less power. The total power dissipated during the overvoltagecondition, represented by curve 68, remains relatively constant.

FIG. 20 depicts a preferred overvoltage, overcurrent protection circuit69 including a PTC device 62 in series, and a shunt regulator 50 inparallel, respectively, with a battery cell 24. As indicated by arrow71, the PTC device 62, which can be a Raychem model VTP210 PTC device,is thermally coupled to the shunt regulator 50. The shunt regulator 50comprises a MOSFET switch 51, op amp controller 53, precision referencevoltage 55 and voltage divider 75, all formed on a single silicondevice.

In particular, the MOSFET 51 is controlled (i.e., turned ON or OFF) bythe op amp 53, which outputs a voltage signal to activate the gate ofthe MOSFET 51 upon detecting that the voltage across the battery cell 24has reached a specified threshold level. Towards this end, the positiveterminal of the op amp 53 is coupled to the voltage divider 75, whichcomprises a pair of resistors 57 and 59 in parallel with the batterycell 24. The negative terminal of the op amp 53 is coupled to theprecision reference voltage 55, which in turn is connected to thenegative (ground) terminal of the battery cell 24. By sizing thevoltage-bridge (i.e., resistors 57 and 59) the upper voltage thresholdis determined. In a preferred embodiment, the resistors 57 and 59 aretrimmed for precision accuracy

In the event the voltage across the battery cell 24 rises to thethreshold level, the gate of the MOSFET 51 is activated. As currentstarts to conduct through the MOSFET 51, the voltage across the batterycell 24 is limited, thereby clamping the voltage. The gate of the MOSFET51 is modulated to maintain the output voltage level. As describedabove, the current passing through the MOSFET 51 heats the silicon shuntregulator 50, which in turn heats the PTC device 62. As soon as the PTCdevice 62 reaches its switching temperature, the current across thecircuit, and thus across the shunt regulator 50, is choked backsubstantially. This reduces the heat generated by the shunt regulator50, whereby the circuit 69 will ultimately operate at its thermalequilibrium, with the current choked by the PTC device 62 and thevoltage clamped by the shunt regulator 50. Because the voltage isclamped by the shunt regulator 50, the current level can rise rapidly.Preferred techniques for controlling the current and voltage through theregulator are disclosed and described below in conjunction with furtherpreferred embodiments.

FIG. 21 depicts an alternate preferred embodiment of the protectioncircuit, 69′, wherein the op amp and voltage reference 55 are formed ona separate silicon device 72 from the MOSFET 51 and voltage bridge 75.For example, a suitable combined reference 55 and op amp controller isthe LTC1541 model controller by Linear Technologies Corporation.

Referring to FIG. 22, the regulator circuit 69 (or 69′) is, in effect, aprecision clamping device, which drives the MOSFET 51 to regulate thevoltage across the battery cell 24. Of course, the voltage reference 55requires some amount of current for operation. As will be appreciated bythose skilled in the art, the more precision the reference 55, the morecurrent is needed for its operation. Although the amount of current isstill relatively small in a typical application, e.g., in the micro-amprange, this current draw may exceed the desirable leakage level for thebattery cell 24. This is especially a concern in that the shuntregulator 50 is only activated when the voltage level exceeds its normaloperating range.

Referring to FIG. 23, in order to minimize the leakage current needed tooperate the precision voltage reference 55, a further activation circuit80 may be employed to selectively activate the regulator control circuit55/53 via a second MOSFET switch 81. In particular, the activationcircuit 80 employs a much less precise voltage detection means (notshown) than does the shunt regulator 50, but in return has a much lowerleakage current. Only when the voltage across the battery cell 24reaches a level approaching the maximum allowable level will theactivation circuit 80 turn ON MOSFET switch 81, thereby activating theregulator control circuit 55/53. Because the battery circuit will nearlyalways be operating below the maximum allowable voltage, the relativelyhigher leakage of the precision voltage reference 55 is not an issue.

Thus far, the described methods and devices have been for purposes ofprotecting against an overvoltage or overcurrent condition. However, itmay also be desirable to protect rechargeable elements, such asrechargeable battery cells, against an undervoltage condition, i.e., dueto an overdischarge.

Towards this end, FIG. 24 shows a preferred over or undervoltageprotection circuit 100 employed between a charger 101 and a rechargeablebattery cell 124. The protection circuit 100 generally includesovervoltage protection circuit 102 connected in parallel with anundervoltage protection circuit 103. The overvoltage protection circuit102 includes a PTC device 104 in series with the battery cell 124 and ashunt regulator 105 in parallel with the battery cell 124. The shuntregulator 105 includes an op amp controller 110 driving an n-channelMOSFET 114. The op amp 110 is connected at its positive input 128 toresistors 106 and 108 in a voltage divider configuration. The resistors106 and 108 are connected in series, following the PTC device 104,between the high and low terminals of the battery charger 101 and cell124. The negative input 126 of op amp 110 is coupled to a negative inputterminal 130 of a comparator 120 in the undervoltage protection circuit103. A reference voltage 140 couples the respective negative inputterminals 126 and 130 to ground.

The output of op amp 1is connected to the gate of the MOSFET 114. Thedrain and the source terminals of the MOSFET 114 are connected to therespective high and low potentials of the battery 124 and charger 101.The comparator 120 of the undervoltage protection circuit 103 isconnected at its positive input 132 to resistors 116 and 118 in avoltage divider configuration. The resistors 116 and 118 are connectedin series between the high and low terminals of the battery charger 101and cell 124. The output of the comparator 120 is connected to the gateof an n-channel MOSFET 122, whose source and drain terminals areconnected in series across the low (ground) terminal of the battery cell124.

The op amp 110 monitors the potential difference between its positiveand its negative inputs, and drives the output accordingly. For anoperational amplifier, if the voltage at its positive input is greaterthan that of the negative input, the operational amplifier output isdriven High. If the voltage at the positive input is lower than that ofthe negative input, the output of the operational amplifier is drivenLow. The negative input is connected to the precision voltage reference140. The resistors 106 and 108 provide a divider bridge, which allows adesigner to choose the overvoltage limit.

In a preferred embodiment, the resistors 106 and 108 are selected tomake the voltage at the positive input 128 of op amp 110 equal thereference voltage when the voltage across the cell 124 reaches aspecified threshold. During an overvoltage fault condition, the voltageacross the cell 124 exceeds the threshold and the voltage at thepositive input of the op amp 110 becomes higher than the voltagereference 140. The op amp 110 amplifies this voltage difference betweenits positive input terminal 128 and negative input terminal 126 andprovides an amplified signal at its output terminal 134, which switchesON MOSFET 114.

As the MOSFET 114 conducts current, the voltage across the battery cell124 is clamped and effectively drops. The voltage at the positive input128 of op amp 110 reduces accordingly, as does the output 134. Thereduction of the output 134 of the op amp 110 causes the in-pathresistance R_(DS)-ON of the MOSFET 114 (which is, effectively, avariable resistor) to increase. This increase in R_(DS) in turn forcesthe voltage at the positive input 128 of the op amp 110 to increase.This alternating reduction and increase in voltage seen at input 128continues until equilibrium is reached where the output voltage drivingthe gate of MOSFET 114 is such that the voltage across resistor 108 isequal to the voltage reference 140.

For the overvoltage condition, MOSFET 114 is ON and the shunt regulator105 dissipates energy, which is thermally transferred to the PTC device104. As described above, when the temperature of the PTC device 104reaches its trip temperature, its resistance will dramatically increase,thereby choking the current flowing through the MOSFET 114. Powerdissipation is then shared between the shunt regulator 105 and PTCdevice 104, protecting the MOSFET 114 from failure due to excessivetemperature.

The undervoltage circuit protection 103 works in a manner somewhatsimilar to that of the overvoltage protection circuit 102. The negativeinput 130 of the comparator 120 is connected the voltage reference 140.The positive input of the comparator 120 is connected to a dividerbridge, involving resistors 116 and 118, which monitors the voltageacross the cell 124 and effectively sets an undervoltage limit. Theoutput 138 of the comparator 120 drives the gate of the N Channel FETtransistor 122, connected in series with the load.

Under normal operation, the voltage across the cell 124 is above theundervoltage limit, and the voltage at the positive input pin 132 of thecomparator 120 is greater than voltage reference. Thus, the output 138of the comparator 120 is driven High, and transistor 122 is ON, allowingthe cell 124 to discharge through a load. When the voltage of the cell124 drops below the pre-selected undervoltage limit, the output 138 ofthe comparator 120 is driven Low, the transistor 122 turns OFF and thecell 124 is disconnected from the load. Charging of the cell 124 is nownecessary to disable the undervoltage protection. Once the voltageacross the cell rises above the undervoltage limit, transistor 122 turnsback ON and discharging is allowed.

The output of the op amp 110 is driven high during an overvoltage faultand consequently turns ON the N-channel FET 114. While an N-channel FETis described, it is possible to use an op amp, which will provide a lowoutput during a fault and drive the gate of a P-channel FET. Similarly,it is also possible to use a comparator that would provide a low outputvoltage during an undervoltage and have it drive the gate of a high sideP-channel FET connected in series with the load. The configuration ofthe op amp 110, comparator 120 and MOSFETs 114 and 122 is flexible.Additionally, the battery pack designer is free to choose theovervoltage and undervoltage limits to satisfy any application.

For cell charging circuits, it is desirable to avoid reverse batterycharging or reverse charge build-up. Reverse battery charging occurswhen undesirable currents flow in a direction opposite to that necessaryto charge a battery cell. Reverse currents not only decrease theefficiency of a charging circuit, but may also cause damage to thebattery cell. An advantage of the preferred protection circuit 69 isthat its current limiting properties will also serve to choke backharmful reverse current flow.

With reference to FIG. 25, the build-up of any reverse current passingby the shunt regulator MOSFET 51 will be conducted through its bodydiode 148. In particular, the passing of sufficient reverse current willcreate a heat path through diode 148, thereby generating ohmic heatingof the shunt regulator device 50 due to the power dissipated by thecurrent. As described in detail above, the heat is conducted through thethermal path (indicated by arrow 151) from the shunt regulator 50 to thePTC device 62, until the PTC device reaches its switching temperatureand trips, thereby substantially choking back on the reverse current.

Should additional protection be desired, e.g., for situations wherelarge power dissipation through the body diode 148 is undesirable,further diode resistance 152 (shown in FIG. 26) is preferably in serieswith body diode 148 (i.e., within the MOSFET silicon), in order togenerate heat when conducting reverse currents. The additional heatgenerated helps to create the heat path to the PTC device 62 withoutrelying solely on the body diode 148 and, importantly, without requiringthe diode 148 to dissipate as much power to trip the PTC device 62. Ineffect, the resistive path through diode resistance 152 can extend thereliability and expected life of the MOSFET 51. Further, since theresistance 152 can generate heat faster than the body diode 148 alone,the heat path to the PTC device 62 is generated more rapidly.

FIG. 27 is a cross-sectional view of a preferred MOSFET device 170 foruse as the shunt regulator with the added series resistance in theprotection circuit of FIG. 26. The MOSFET 170 is designed so that itschannel 164 will generate heat. In particular, a PTC device 156 iscoupled to N-type silicon 166 by a lead frame 158. The diode/resistorcombination 148 and 152 of FIG. 16 can be implemented as a p-n junctionwhere the resistance of diode can be determined by a body path in atransistor. To make the body path more resistive, the path is madelonger. To increase the body path resistance, a body contact 160 isplaced away from the channel 164. If the body-to-drain diode is forwardbiased, as is the case with the configuration of the respective sourceand drain terminals 162 and 163 in device 170, the P-drift region 168will heat up when conducting current.

Without the added resistance to the diode in a conventional MOSFET, aforward-biased diode can destroy the package before sufficient heat isgenerated to trip the PTC device. By employing a more resistive diode,more heat can be generated as necessary to create the heat path toconduct away reverse currents. A more resistive diode configurationoffers a higher breakdown point while, in some embodiments, allowing theheat path to be generated more rapidly. An additional advantage over“smart” semiconductor devices is that, after failure, a PTC device willstill be in place to interrupt potentially damaging current flow.

In accordance with a still further aspect of the disclosed inventions,FIGS. 28 and 29 show preferred semiconductor devices for protectingagainst reverse battery discharge,—i.e., for limiting channel current toan acceptable level in the “reverse” direction, while presenting littleohmic resistance in the “forward” direction.

More particularly, FIG. 28 depicts a diffused structure 450 having a topcontact 451 and bottom contact 452 connected to a JFET region 454. Thetop contact is connected to a metal, or ohmic contact, 453. When the topcontact 451 is positively biased relative to the bottom contact 452, alarge current will flow through the JFET region 454. This positive biasis shown as curve segment 458 in FIG. 30. If the voltage is reversed(i.e., negative top contact 451 relative to bottom contact 452), thecurrent passing through the JFET region 454 will initially have an ohmicbehavior.

The reverse bias is shown as curve segment 457 in FIG. 30. As thereverse current increases, the pn junction between regions 455 and 456becomes increasing reverse biased. The reverse bias creates a depletionregion and further obstructs current flow through the JFET region 454.This restriction is current flow limits the maximum value of the reversecurrent to a manageable level.

FIG. 29 depicts a trench structure 460 having a top contact 461 andbottom contact 462 connected to a JFET region 464. The top contact isconnected to a metal, or ohmic contact 463. When the top contact 461 ispositively biased relative to the bottom contact 462 a large currentwill flow through the JFET region 464. This positive bias is shown ascurve segment 458 in FIG. 30. If the voltage is reversed (i.e., negativetop contact 461 relative to bottom contact 462) the current passingthrough the JFET region 464 will initially have an ohmic behavior. Thereverse bias is shown as curve segment 467 in FIG. 30. As the reversecurrent increases, the gate effect creates a depletion region inside thetrench channel 465. The depletion region further obstructs current flowthrough the region 454 and limits the maximum value of the reversecurrent at a manageable level.

With reference again to FIG. 16, the current limiting PTC device 62 andshunt regulator 50 work in tandem to protect the battery cell 24 fromovercharging or being exposed to overvoltage conditions. When anovervoltage condition occurs, excess power must be dissipated away fromthe shunt device 50 (in particular the MOSFET 51), so as to avoid anydamage or shorting of the device.

In particular, in order to protect the battery cell 24, the shuntregulator device 50 must be able to withstand significant current surgesuntil the PTC device 62 trips. One, previously discussed, approach tominimize this exposure is to design the PTC device 62 to trip atrelatively low temperatures. There are limits to this approach, however,in that the PTC device 62 must allow for sufficient current conductionduring normal operation (i.e., charging or discharging) of the batterycell, without tripping due to internal ohmic heating.

Generally, by being able to use devices in a protection circuit thatdoes not have to withstand high power or high voltages, less costlydevices can be used, or circuits requiring less topology (i.e.,“silicon”) may be employed.

As has been described herein, as current flows into the regulator, theregulator heats and raises the resistance of the PTC device to limitcurrent in the regulator. The current in the regulator and PTC devicestabilizes at a value such that the power dissipation in the respectivedevices is enough to keep the PTC device on the steep portion of itsresistance versus temperature (i.e., “R(T)”) curve (e.g., 1 to 1.5 wattswhen using a Raychem VTP210 PTC device). This technique is adequate toprevent damage to the regulator during relatively small to moderateovercurrent conditions.

For higher power transient events where the current can reach largevalues, however, the delay for the heat from the regulator to assist intripping the PTC device is determined by the thermal time constants forthe heat to flow into the PTC device. Due to this lag, the silicon ofthe regulator can reach very high temperatures where it can possibly bedamaged before the PTC device trips. While the regulator silicon can beincreased in size to handle large surges of current, this addssignificant cost to the device.

FIG. 31 depicts a protection circuit for a cell 180 using a shuntregulator 182 and PTC device 184. The voltage across the cell 180 andthe shunt 182 must be identical since the devices are in parallel.Unlike the ideal situation, in practice, a clamp region of the devicecannot be absolutely vertical, as is illustrated in FIG. 32. For currentto flow in the regulator, the voltage across it must increase a smallΔV. As this small ΔV also appears across the cell 180, the cell 180 willtry to charge and draw some current from a supply 186. This addedcurrent flows through, and will assist in tripping, the PTC device 184.The larger the ΔV, the greater the current flowing into the cell 180and, therefore, the quicker the PTC device 184 will trip via the thermallink 188.

For transient conditions, the cell 180 can be envisioned as a voltagesource (or very large capacitor) with a series resistance equal to theinternal resistance of the cell 180. The voltage of the source (orcapacitor) is equal to the voltage of the cell 180 before the transientoccurred. As an example, if a cell has an internal impedance of 0.1 Ω,the cell would draw 10 additional amps through the PTC device if thevoltage increased by a ΔV of 1 V. The current through the regulatorwould be the current on the I-V curve (FIG. 32) at the higher voltage.At the higher voltage, the total current will increase through the PTCdevice 184, which would trip much quicker than in a situation where nocell 180 is present.

Having the regulator with a shallow slope in a clamp region protects thesilicon under transient conditions, but has a negative impact on theperformance of the cell 180 during slow moving faults. A problem canarise if the voltage is increased very slowly, since increasing thevoltage slowly allows the cell 180 to “trickle charge” and a large ΔVacross the cell will not be present as the voltage increases. The cellvoltage will track the I-V curve of the regulator, until the regulatorconducts enough current to heat and trip the PTC device. In some cases,however, the extra voltage build-up will unacceptably degrade cellperformance or damage the cell 180. In practice, for optimalperformance, the I-V characteristic must be as steep as possible toprevent the cell 180 from overcharging due to a “trickle charge”.

Thus, there appear to be two requirements. On one hand, high faulttransients require large silicon or shallow sloped clamp regions. On theother hand, slow moving faults usually require much steeper clampregions. Ideally, the solution is to make a device, which will have theI-V characteristic as shown in FIG. 33. For low current fault events,such as is the case for a slowly rising voltage, the device acts as aclamp and prevents the voltage from increasing past the clamp voltage.The minimum current required through the device in order to cause thePTC device to trip would reside on the steep portion 190. For slowmoving faults with low potential current, the device would operate justas a clamp with a very steep clamp region 190. For faults with largercurrents, the clamp limits the current to a set value 192 and allows thevoltage to increase. With a cell attached, this increase in voltage willdraw a large current from the cell and assist in tripping the PTCdevice. Once the PTC device has tripped, the voltage across theprotection device and cell will be reduced and the maximum voltageacross the cell and device that can be obtained will be the voltage atthe steep portion 190. Notably, since the minimum current required totrip the PTC is on the steep portion of the curve, a sustainedovervoltage condition cannot be obtained.

In the preferred protection circuit of FIG. 34, an operational amplifier200 monitors the potential difference between its positive and itsnegative inputs 202 and 204, respectively, and drives the output 206accordingly. If the voltage at its positive input 202 is greater thanthat of the negative input 204, the operational amplifier output 206 isdriven High. If the voltage at the positive input 202 is lower than thatof the negative input 204, the output 206 of the operational amplifier200 is driven Low. The negative input is connected to a voltagereference 208. The resistors 210 and 212 provide a divider bridge, whichallows a designer to select any overvoltage limit for the cell 222.

In particular, the operational amplifier 200 adjusts the gate voltage onthe FET 214 to force the device to have a clamp IV characteristic. FIG.35 depicts a family of I_(d)-V_(ds) characteristics for an n-channelFET. As shown, V_(gs) can be adjusted along vertical slope 216 to obtainclamp performance. Depending on the gate-source voltage, the draincurrent can take any value at a particular drain-to-source voltage.

To obtain the desired characteristic, the voltage at the gate 206 of theFET 214 can be set to not exceed a set value. This can be done, forexample, by clamping the gate voltage, as shown in FIG. 36.

In particular, FIG. 36 depicts the same circuit as FIG. 34, except thata zener diode 220 is attached between the output 206 and ground. Byintroducing the zener diode 220, the voltage at the gate of the FET 214is limited. The zener diode 220 effectively clamps the gate voltage ofthe FET 214, allowing the voltage across the battery 222 to increase anddirecting more current to the battery 222. As shown in FIG. 37, V_(gs)can be adjusted to obtain clamp performance and/or clamp the voltage at206, so that the FET operates in saturation.

A key to this approach is that the circuit actually directs power to abattery cell and away from a FET 214. Such an approach recognizes thatin some configurations a battery 222 can easily absorb some additionalvoltage and/or current from 206 for a set duration. By absorbingadditional voltage and/or current, the FET is protected. Because of thereduced performance requirements on the FET 214, a less costly FET, orone taking less space, can be used.

The PTC device 224, in series with the battery 222, will also see thehigher currents. As the higher currents are exposed to the PTC device224, the PTC device will trip more quickly and advantageously dissipatepower. In this way, while the battery will be exposed to higher voltageand currents, the PTC device will trip to dissipate power before thebattery 222 is exposed to any power levels, which may damage thebattery. Simply, any high currents seen by the battery 222 also pass thePTC device 224; these higher currents trip the PTC device before thebattery 222 is exposed to any damaging power levels.

While the I-V characteristics shown in FIG. 37 represent oneconfiguration, further optimization may be obtained by changing the I-Vcharacteristic even further after the vertical section 226. Possible I-Vcharacteristics are shown in FIG. 38. By reducing the current after theclamp region, the power dissipated in the device is reduced during thetransient event and may result in further silicon size reductions.

The above approach takes advantage of the fact that the battery 222 canwithstand some additional current and voltage levels before the PTCdevice trips. As in any embodiment disclosed herein, in order to assurethat the battery 222 is not exposed to damaging power levels caused byextreme voltage and/or current levels or by failure of the regulator, athermal fuse or regular fuse can be used to isolate the battery.

Referring to FIG. 39, a preferred battery protection circuit is embodiedin a three terminal battery protection device 229, which generallycomprises a PTC device 236 thermally coupled with a MOSFET regulatorswitch 232. In particular, a first terminal 231 of the protection device229 couples to a positive lead of an external charging source ordischarging load (not shown), to an input terminal of the PTC device236. A second terminal 233 of the protection device 229 couples both anoutput terminal of the PTC device 236 and a drain terminal of the MOSFETregulator 232 to the positive terminal of a battery (not shown). A thirdterminal 235 of the protection device 229 couples the negative terminalof the battery to the source of the regulator 232 and to the groundterminal of the charging source or discharging load.

As indicated by arrow 234, the drain terminal of the regulator 232 formsboth a thermal and electrical link to the PTC device 236. A fuse 230such as, e.g., a bond wire or solder bond, is placed in series with thesecond terminal 233 in order to provide a last measure of protection tothe battery.

For purposes of better illustrating still further aspects of theinventions disclosed herein, variations of device 229 are now described.

Referring to FIGS. 40 and 41, in lieu of (or in addition to) fuse 230,an additional PTC device 237 can be added in series with the MOSFETregulator 232 to provide further protection should the regulator 232fail and short circuit. Notably, the additional, or “parallel” PTCdevice 237 may be coupled to either the source (FIG. 40) or drain (FIG.41) of the MOSFET regulator 232. In a preferred embodiment, the parallelPTC 237 device is configured to trip before the current passing throughit and the regulator 232 generates sufficient ohmic heat to cause theregulator 232 to fail and short circuit.

Although the parallel PTC device 237 would not increase the pathresistance seen by a battery 222 being protected by the device 229, ifthe parallel PTC device 237 were to inadvertently trip due to excessthermal heat or otherwise fail open during normal operation of thebattery 222, the regulator 232 would no longer be coupled across thebattery 222. One approach to minimize the chance of this occurring isfor the parallel PTC device 237 to have a higher transition temperature(i.e., higher threshold trip current) than the “series” PTC device 236,to insure that the series PTC device 236 will trip before the parallelPTC device 237. In this scenario, the parallel PTC device 237 stillplays a protection role with respect to preventing further dischargefrom the battery 222 through the regulator 232, after the series PTCdevice 236 has tripped.

As shown in FIG. 42, with the added parallel PTC device 237, the MOSFETregulator 232 is preferably configured to sense the voltage across thebattery path via path 239, without the added resistance of the parallelPTC device 237.

Turning now to still further aspects of the present inventions disclosedherein, it is generally known that lithium-ion batteries should beprevented from being overdischarged. FIG. 43 represents a typical priorart circuit 470 employed for preventing overdischarge of a battery 472.In particular, a FET 471 having a gate resistance 473 is connected inseries with the battery 472 and a load 474. Once a preset low batteryvoltage is reached, the FET 471 will automatically turn OFF, therebypreventing the battery 472 from further discharging across the load 474.However, during the battery discharge process, the FET 471 is subjectedto relatively high power dissipation, which may increase its temperatureabove acceptable limits. In particular, this thermal stress can damagethe FET 471, such that the overdischarge protection circuit 470 mayfail.

Referring to FIG. 44, a preferred overdischarge protection circuit 480,which also provides protection in case of overcharging, employs a FETdevice 481 in series with a battery 482 and load 483. In particular, theFET 481 has its source terminal 488 coupled to the output of the load483 and its drain terminal 489 coupled with the negative terminal of abattery 482. The positive terminal of the battery 482 is coupled to theinput of the load 485, so that, when the FET 481 is ON, the battery 482will discharge through the load 483.

A PTC device 484 is inserted in the reverse discharge protection circuit480 in parallel with the respective battery 482 and load 483. The gateterminal 487 of the FET device 481 is coupled in a divider configurationthe PTC device 484 and a resistor 485. The respective (low temperature)resistances of the PTC device 484 and resistor 485 are sized such thatthe voltage seen at the gate terminal 487 of the FET device 481 willkeep the device ON, so long as the voltage stays above the fulldischarge level of the battery 482. In one preferred embodiment, for atypical rechargeable lithium battery pack, the low temperatureresistance of the PTC device 484 is selected at about 10 kohms, and thevalue of the resistor 485 is 1 Mohms.

In accordance with this aspect of the inventions disclosed herein, thePTC device 484 is thermally coupled to the FET device 481 as a furtherprotection against failure of the FET device 481 in case of overchargingof the battery 482. As the voltage across the PTC device 484 and, thus,the FET device 481, approaches a level that might otherwise cause theFET device 481 to fail, current flowing through the PTC device 484 willsufficiently heat the device 484 to its trip temperature. Once PTCdevice 484 trips to it's high resistance state, the voltage across thedevice 484 will immediately drop to a level below the threshold gatevoltage of the FET device 481, causing the FET to turn OFF.

In a preferred embodiment, the PTC device 484, FET device 481 andresistor 485 are sized such that the PTC device 484 will trip to highresistance and shut off the FET device 481 well before a failure of theFET device 481 is possible due to the rising current caused by anovercharging condition. By way of example, in a preferred embodimentemployed for protection of a rechargeable lithium battery, the PTCdevice 484 has a (non-tripped) resistance value about 10 kohms andresistor 483 has a value about 1 Mohms. Notably, the protection circuit480 can be optimized for various configurations and FET characteristicsby modifying the ratio of resistors 484 and 485.

In accordance with further aspects of the inventions disclosed herein,rechargeable battery protection devices and preferred methods for theirmanufacture and assembly will now be described.

Referring to FIGS. 45-47, a preferred three terminal battery protectiondevice 240 includes a PTC chip 242, which is thermally and electricallycoupled to a MOSFET regulator 244.

The PTC chip 242 includes a layer of PTC material 246 having a firstmetal electrode layer 248 covering a first side, and a second metalelectrode layer 250 covering a second (i.e., opposite) side. The metalelectrode layers 248 and 250 are respectively coated with an insulatingfilm 249 and 251. A portion of the insulating film 251 is missing at oneend of the PTC chip 242, exposing a portion of the metal electrode layer250, which forms a first terminal 262 of the protection package 240. Arectangular window 252 is provided in the insulating film 249 proximatethe opposite end of the chip from first terminal 262, exposing a portionof the metal electrode layer 248 upon which the regulator 244 isattached by a solder bond 253.

In accordance with the protection circuit 69 of FIG. 16, the regulator244 includes a MOSFET switch and precision control circuitryincorporated on a single silicon die 254. The die 254 is attached to aheat sink 256, which is electrically coupled to the drain terminal ofthe MOSFET switch. The heat sink 256, in turn, is electrically andthermally coupled to the metal electrode layer 248 of the PTC chip 242via the solder bond 253. The heat sink 256 is also electrically coupledto an external lead 258 extending away from the regulator 244 and overan end of the PTC chip 242 opposite electrode terminal 262. A sourceterminal of the MOSFET switch is electrically coupled to a secondexternal lead 259 extending away from the regulator 244 adjacent to, andin the same manner as, lead 258. Leads 258 and 259 form respectivesecond and third terminals of the protection device 240.

When the protection device is employed with a rechargeable battery (notshown), the first terminal 262 is coupled to the positive terminal of abattery charging device or discharging load device. The second terminal258 is coupled to the positive terminal of the battery, and the thirdterminal 259 is coupled to the both the negative terminal of the batterypack and the negative terminal of a battery charging device ordischarging load device. With this arrangement, an electrical path isformed from the first terminal 262 to the second terminal 258 of thepackage 240 via the metal electrode layer 250, PTC material 246, metalelectrode layer 248, solder bond 253, and heat sink 256, respectively.If the MOSFET channel is activated (i.e., during an overvoltagecondition), an electrical path is also formed from the first terminal262 to the third terminal 259 via the metal electrode layer 250, PTCmaterial 246, metal electrode layer 248, solder bond 253, heat sink 256,and MOSFET switch channel, respectively.

Attachment of the regulators 244 to the metal electrode 248 of the PTCchip 242 for assembly of device 240 may be readily incorporated into aknown process for manufacturing the PTC chips 242. In particular, PTCmaterial 246 is composed according to the desired performancecharacteristics, e.g., conductivity, tripping temperature, etc., andthen formed into a sheet of a desired thickness depending, again, on thedesired performance characteristics, e.g., thermal mass. The metalelectrode layers 248 and 250 are provided as thin foil sheets of, e.g.,nickel, copper or an alloy, which are pressed onto respective top andbottom surfaces of the sheet of PTC material 246. The insulating filmlayers 249 and 250 are silk screened over the respective metal layers248 and 250. The layer is selectively photo-masked, and then exposed tolight. The unexposed material masking material is then removed to exposeportions of the metal layers to be used as respective electrodeterminals for the PTC chips 242.

The sheets are then cut into multiple PTC chips 242 of selecteddimensions. More specific details of preferred PTC device manufacturingprocesses and methods are disclosed in U.S. Pat. Nos. 5,852,397 and5,831,150, which are fully incorporated herein by reference for all thatthey teach.

As part of the masking step in the above-described manufacturingprocess, the windows 252 in the insulating film layer 249 may be formedin the respective PTC chips 242 in any suitable shape. By way ofexample, FIG. 48 shows a sheet 270 of the PTC material 246 at the pointwhere the metal electrode layer 248 and insulating film 249 have beenapplied. A pattern 271 is shown in sheet 270 for demarcation of therespective individual chips 242. Windows 252 are formed in theinsulating film 249 of each PTC chip 242 to expose a portion of themetal electrode layer 248. The windows 252 are, in effect, respectivepad locations for mounting the regulator devices 244.

Towards this end, solder material 253 is deposited onto the exposedmetal electrode layer 248 in each window 252 and the respective heatsinks 256 of the regulator devices 240 are placed on the solder material253. The PTC sheet 270 is then exposed to sufficient heat to re-flow thesolder material 253. The windows 252 are preferably sized so that,during the re-flow process, the respective heat sinks 256 will “selfcenter” within the window 252. Once the re-flow process is accomplished,the individual devices 240 are cut from the sheet 270 along lines 271.As will be apparent to those skilled in the art, the above-listed orderof manufacturing steps is but one possible approach, and other sequencesmay be alternately employed without departing from the inventiveconcepts taught herein. By way of example, it may be desirable to cutthe individual regulator devices 240 from the sheet prior to performingthe solder re-flow.

Referring to FIGS. 49 and 50, a preferred process for manufacturinghousings for the three terminal devices 240 by employing an injectionmolding process is as follows:

A lead frame 300 made from a flexible, conductive metal, such as, e.g.,copper, nickel or aluminum, comprises a pair of parallel frame edges 302and 303 that are configured to be advanced into an injection moldingmachine (not shown). Spaced holes 310 are provided along the frame edgesfor alignment (or registration) of the lead frame 300. The respectiveframe edges 302 and 303 are separated by spaced cross support members301, which serve to both evenly space the frame edges 302 and 303, andto divide the lead frame 300 into evenly spaced, repeating sections 305.

A first tab 304, preferably made of the same flexible metal as the leadframe 300, extends from the frame edge 302 into each section 305.Likewise, second and third tabs 306 and 308, also preferably made of thesame flexible metal as the lead frame 300, extend substantially parallelto one another from the frame edge 303 into each section 305. Inparticular, the respective tabs 304, 306 and 308 are configured in apattern to allow for bulk assembly of injected molded housings 314 forthe three terminal protection devices 240. The tabs 304, 306 and 308 arepreferably resilient and bendable to form electrical terminals ofvarious sizes and configurations.

As seen in FIG. 50, a device housing 314 is formed around the tabs 304,306 and 308 in each section 305 of the lead frame 300, wherein a distalportion of each tab 304, 306 and 308 is exposed inside the housing 314.In accordance with known injection molding techniques, a plurality ofdevice housings 314 may be simultaneously formed. Prior to the injectionprocess, the distal ends of tabs 304, 306 and 308 may be crimped or bentto best position the respective ends for making electrical contact witha device 240 placed into the finished housing 314. Such bending orcrimping may also serve to add strength to the end walls of the housing314.

Referring to FIG. 51, once the housings 314 have sufficientlysolidified, the respective frame edges 302 and 303, and cross-supportmembers 301 are removed, and an assembled three terminal device 240 isplaced into each housing 314. In particular, the devices 240 are placedinto the housings 314 such that the first, second and third terminals262, 258 and 259 make electrical contact with the exposed distal ends oftabs 304, 306 and 308, respectively. Alternatively, PTC chip 242 andregulator device 244 can be placed into each housing 314 to obtain thesame functionality. The terminals 262, 258 and 259 may be bonded (e.g.,by a solder bond) to the respective tabs 304, 306 and 308, or mechanicalcontact may be relied on. If mechanical contact is relied on for therespective electrical connections, however, tabs 304, 306 and 308 shouldbe sufficiently resilient to provide an internal spring force biasedagainst the respective terminals 262, 258 and 259.

As seen in FIG. 52, a non-electrically conductive cover 315 is thenmolded, or otherwise bonded, over the opening of the housing 314, toboth secure and isolate the device 240. The housings 314 are preferablysized so as to snuggly accommodate the devices 240. Importantly,however, the housings must not be too confining, or otherwise exertcompressive force on the device 240, as the PTC chips 242 must beallowed to expand (e.g., up to approximately 10%) when heated in orderto operate correctly.

Once the device 240 is sealed in a respective housing 314, the tabs 304,306 and 308 become the respective leads—i.e., with tab 304 configuredfor coupling to the positive terminal of a battery charging device ordischarging load device; tab 306 configured for coupling to the positiveterminal of a battery; and tab 308 configured for coupling to both thenegative terminal of the battery and the negative terminal of therespective charging device or discharging load device. The flexibilityof the tabs 304, 306 and 308 provides for ease in attachment to therespective positive and negative battery terminals (e.g., by a spotweld), as well as to electrical connectors for attaching to a chargingdevice or discharging load.

In alternate embodiments, the devices 240 can be potted, rather thanplaced, in the housings 314. Depending on the desired performancecharacteristics, the selected potting material be either thermallyconductive or thermally insulating.

As will be apparent to those skilled in the art, the injection moldedhousings 314 can be alternately formed from various non-electricallyconductive materials, such as plastics or ceramics. The characteristicsof the selected material, as well as the housing dimensions (i.e.,thickness) should be selected based on such factors as cost,availability, “moldability” (i.e., how rapidly the material solidifiesafter being injected), strength and thermal conductivity, among otherfactors. Such design considerations also include device installationrequirements and reworking requirements. It is important, however, thatthe housing material not materially interfere with the tripcharacteristics of the PTC chip 242 of the devices 240.

In particular, as discussed above, the trip time is the amount of timeit takes for a device to switch to a high-resistance state once a faultcondition has been applied through the device. If the packaging materialhas a thermal conductivity that is too low, the PTC device 242 may overheat under normal operating conditions, causing undesired (“nuisance”)tripping to occur. On the other hand, if the packaging material is madefrom material having thermal conductivity that is too high, the PTCdevice 242 may trip too slowly in an overvoltage, or overcurrentcondition.

The selection of the housing material and dimensions should also takeinto account the expected application or environment in which the device240 will be operating. Design considerations typically include expectedvoltage and current operating conditions, surge current ratings, maximuminternal battery pack operating temperature during normalcharge/discharge, and the range of expected ambient operatingtemperatures.

One general advantage of the three terminal devices 240 is that they canbe thermally coupled to a battery pack to thereby also provideovertemperature protection. If the devices 240 are placed in thehousings 314, then the ability to achieve a thermal heat path from thebattery pack to the device 240 must also be taken into account.

FIGS. 53-55 depict alternative lead frame configurations 320 and 320′for use in the above-described process for manufacturing housings forthe three terminal devices 240 by an injection molding process.

Referring in particular to FIG. 53, as with lead frame 300, lead frame320 is preferably made from a flexible, conductive metal, such as, e.g.,copper or aluminum. Frame 320 comprises a pair of parallel frame edges322 and 323 that are configured to be advanced into an injection moldingmachine (not shown), via the advancement holes 330. The respective frameedges 322 and 323 are separated by evenly spaced cross support members321, which serve to both evenly space the frame edges 322 and 323, andto divide the lead frame 320 into evenly spaced, alternating sections325 a and 325 b. In particular, sections 325 a and 325 b are mirrorimages of each.

In section 325 a, a first conductive tab 324 extends from frame edge322, a second conductive tab 326 extends from frame edge 323, and athird conductive tab 328 extends from the cross support member 321,respectively. In section 325 b, tab 324 extends from frame edge 323, tab326 extends from frame edge 322. Notably, the conductive tab 328 stillextends from the cross support member 321. In particular, in lead frame320, every other cross support member 321 attaches to the respectivetabs 328 for the adjacent sections 325 a and 325 b, with the remainingevery other cross support members 321 having no attachments.

Referring also to FIG. 55, in a second alternate preferred lead frame320′, the respective sections 325′ are not mirror image, but repeatconsecutively. In other words, lead frame 320′ is exactly like leadframe 320, except that every cross support member 321′ supports tab 328for a single adjacent section 325′.

As with tabs 304, 306 and 308 on lead frame 300, tabs 324, 326 and 328are preferably made of the same flexible metal as the lead frame 320. Inparticular, the respective tabs 304, 306 and 308 are configured in apattern to allow for bulk assembly of injected molded housings for thethree terminal protection devices 240. The tabs 324, 326 and 328 arepreferably resilient and bendable to form electrical terminals ofvarious sizes and configurations.

As seen in FIGS. 54 and 55, a device housing 334 is formed around thetabs 324, 326 and 328 in each section 325 a and 325 b of the lead frame320, wherein distal portions of tabs 324, 326 and 328 are exposed insidethe housing 334. Prior to the injection process, the distal ends of tabs324, 326 and 328 may be crimped or bent to best position the respectiveends for making electrical contact with a device 240 placed into thefinished housing 334. In particular, a distal portion 329 of tab 328 iscrimped such that the “bottom” wall of the molded housing 334 encasesand, thus, electrically isolates, all except a very end portion (shownin phantom in FIGS. 54 and 56). As with housing 314, such bending orcrimping may also serve to add strength to the respective end and bottomwalls of the housing 334.

Notably, the completed housing 334 is identical, regardless of whetherlead frame 320 or 320′ is used. A completed housing 334, i.e., with therespective frame edges 322 and 323 and cross-support members 321removed, is illustrated in FIG. 56. As will be appreciated by thoseskilled in the art, the difference between housing 334 and housing 314is that the tab lead 328 coupled to the ground (or negative) terminal259 of device 240 extends from a side of the housing 334, instead offrom an end. This alternate housing configuration allows for flexibilityin ways the three terminal device 240 can be attached to a rechargeablebattery pack.

FIGS. 57-58 depict an alternate preferred embodiment 340 of theabove-described three terminal protection device 240. Like protectiondevice 240, protection device 340 includes a PTC chip 342, which isthermally and electrically coupled to a MOSFET regulator 344. The PTCchip 342 includes a layer of PTC material 346 having a first metalelectrode layer 348 covering a first side, and a second metal electrodelayer 350 covering a second (i.e., opposite) side. The metal electrodelayers 348 and 350 are respectively coated with an insulating film 349and 351.

As in device 240, a portion of the insulating film 351 is missing at oneend of the PTC chip 342, exposing a portion of the metal electrode layer350, which forms a first terminal 341 of the protection package 340.Unlike in device 240, a portion of the insulating film 351 is alsomissing at the other end of the PTC chip 342, exposing a portion of avia 363 of metal electrode layer 348, which forms a second terminal 361of device 340. A rectangular window 352 is provided in the insulatingfilm 349 proximate the same end of the chip as the second terminal 361,exposing a portion of the metal electrode layer 348 upon which theregulator 344 is attached by a solder bond 353.

Like regulator 244 in device 240, regulator 344 includes a MOSFET switchand precision control circuitry incorporated on a single silicon die354. The die 354 is attached to a heat sink 343, which is electricallycoupled to the drain terminal of the MOSFET switch. The heat sink 343,in turn, is electrically and thermally coupled to the metal electrodelayer 348 of the PTC chip 342 via the solder bond 353. A source terminalof the MOSFET switch is electrically coupled to an external lead 345extending away from the regulator 344, the external lead 345 forming athird terminal of protection device 340.

When the protection device 340 employed with a rechargeable battery pack(not shown), the first terminal 341 is coupled to the positive terminalof a battery charging device or discharging load device. The secondterminal 361 is coupled to the positive terminal of the battery and thethird terminal 345 is coupled to the both the negative terminal of thebattery and the negative terminal of a battery charging device ordischarging load device. With this arrangement, an electrical path isformed from the first terminal 341 to the second terminal 361 via themetal electrode layer 350, PTC material 346 and metal electrode layer348. If the MOSFET channel is activated (i.e., during an overvoltagecondition), an electrical path is also formed from the first terminal341 to the third terminal 345 via the metal electrode layer 350, PTCmaterial 346, metal electrode layer 348, solder bond 353, heat sink 343,and MOSFET switch channel, respectively.

Referring to FIGS. 59-62, in accordance with yet another aspect of theinventions provided herein, a flexible printed circuit board (“pcboard”) 350 is provided with an aperture 351 for attaching a MOSFETregulator 354 mounted on a first side 356 of the pc board 350 to a PTCchip 352 mounted to a second (underlying) side 358 of the pc board 350.

As best seen in FIG. 62, the PTC chip includes a layer of PTC material371 having a first metal electrode layer 370 covering a first side, anda second metal electrode layer 372 covering a second (i.e., opposite)side of the PTC layer 371. The metal electrode layers 370 and 372 arecoated with respective insulating film layers 377 and 378. A portion ofthe insulating film 377 underlying the aperture 351 is missing to exposea portion of metal electrode layer 370 upon which a heat sink 396 of theregulator 354 is attached by a solder bond 357.

A further portion of insulating layer 377 is removed proximate one endof the PTC chip 352, exposing a further portion of the metal electrodelayer 370, which is bonded to a first conductive lead 360 on pc boardsurface 358. At the opposite end of the PTC chip 352, a metal path 382couples metal electrode layer 372 to a small electrode area 394 on thesame side as metal layer 370. A gap 380 electrically isolates electrodearea 394 from metal layer 370. The electrode area 394 is bonded to asecond conductive lead 362 on pc board surface 358. In this manner, thePTC chip 352 is anchored to the pc board surface 358 by the bond ofelectrode 370 to surface lead 360 at one end, and the bode of electrode394 to surface lead 362 at the other end.

The regulator is secured to the first side 356 of the pc board 350 byfirst and second leads 355 and 392. In particular, lead 355 is bonded toa first bond pad 390, and lead 358 is bonded to a second bond pad 391,respectively, on pc board side 356. Bond pad 390 is electrically coupledto a third conductive lead 364 on the pc board surface 356. In thismanner, the regulator device is secured to the pc board surface 356 byboth the bonded leads 355 and 392, as well as the solder bond 357between the heat sink 396 to the PTC chip 352. Notably, lead 355 is alsocoupled to the source terminal of the MOSFET switch 398.

When attached to a rechargeable battery pack (not shown), pc board lead362 is configured for coupling to the positive terminal of a batterycharging device or discharging load device; pc board lead 360 isconfigured for coupling to the positive terminal of the battery; and pcboard lead 364 is configured for coupling to both the negative terminalof the battery and the negative terminal of a battery charging device ordischarging load device. An electrical path is formed from lead 362 tothe second terminal lead 360, via the metal electrode layer 372, PTCmaterial 371, and metal electrode layer 370, respectively. If the MOSFETchannel is activated (i.e., during an overvoltage condition), anelectrical path is also formed from lead 362 to lead 364 via the metalelectrode layer 372, PTC material 371, metal electrode layer 370, solderbond 357, heat sink 396, MOSFET switch channel and source terminal 398,and lead 355, respectively.

One advantage of the embodiment of FIGS. 59-62 is that the PTC chip 352is readily mounted directly to the battery pack casing, allowing the PTCchip 352 to provide protection of the battery pack from anovertemperature condition (i.e., through conductive heating from thebattery pack casing).

While preferred circuits, devices and methods for providing overvoltage,overcurrent and/or overtemperature protection to rechargeable elementshave been shown and described, as would be apparent to those skilled inthe art, many modifications and applications are possible withoutdeparting from the inventive concepts herein.

By way of example, while the forgoing described preferred embodimentsare directed mainly to three-terminal device embodiments, it will bereadily apparent to those skilled in the art that the inventionsdisclosed herein may also be embodied in multiple different-numberedterminal and pin geometries.

Further, while the forgoing preferred embodiments are generally directedto protection circuits and systems, the inventive concepts may also beemployed as voltage regulator circuits, e.g., as part of a batterycharger system. As will be appreciated by those skilled in the art, thePTC devices employed in the above described protection circuits wouldnecessarily need to be tuned for frequent temperature tripping andresetting in a voltage regulator application.

What is claimed is:
 1. A protection device for use with a dischargingload device, comprising: a shunt regulator comprising a transistorswitch; a variable resistor having a first terminal, the variableresistor being thermally and electrically coupled to the shuntregulator; a second terminal electrically coupled to the variableresistor; and a third terminal electrically coupled to the transistorswitch, wherein the variable resistor limits current flowing through thetransistor switch during an overvoltage or overcurrent condition beforethe current reaches a predetermined level sufficient to activate thetransistor switch, and wherein an electrical path from the firstterminal of the variable resistor to the third terminal is formed whenthe transistor switch is activated.
 2. The protection device of claim 1,wherein the variable resistor comprises a positive temperaturecoefficient device.
 3. The protection device of claim 1, wherein thevariable resistor comprises a first electrode layer electrically coupledto the second terminal and a second electrode layer electrically coupledto the first terminal.
 4. The protection device of claim 3, wherein thetransistor switch is attached to a heat sink that is electrically andthermally coupled to the first electrode layer of the variable resistor.5. The protection device of claim 1, wherein the variable resistorswitches from a relatively low resistance to a relatively highresistance when heated to a certain transition temperature, and whereincurrent flowing through the transistor switch at the predetermined levelcauses ohmic heat generation in the transistor switch to cause thevariable resistor to substantially reach its transition temperature. 6.The protection device of claim 1, wherein the shunt regulator furthercomprises control circuitry configured to activate the transistorswitch.
 7. The protection device of claim 1, wherein the transistorswitch has a thermally-compensated voltage characteristic.
 8. Theprotection device of claim 1, wherein the transistor switch is a MOSFETswitch having a relatively high resistance, reverse-current body diode.9. The protection device of claim 1, wherein the transistor switchhaving a channel through which current may flow in a forward directionif positive-biased, or a reverse direction if negative-biased, theswitch being constructed such that current flowing in the forwarddirection encounters relatively low resistance, and current flowing inthe reverse direction encounters relatively high resistance.
 10. Theprotection device of claim 1, wherein the shunt regulator is configuredto mount on a first side of a printed circuit board, and wherein thevariable resistor is configured to mount to a second side of the printedcircuit board.
 11. The protection device of claim 10, wherein the firstterminal is bonded to a first bond pad on the second side of the printedcircuit board, the first bond pad being electrically coupled to a firstconductive lead on the printed circuit board.
 12. The protection deviceof claim 10, wherein the second terminal is bonded to a second bond padon either the first side or the second side of the printed circuitboard, the second bond pad being electrically coupled to a secondconductive lead on the printed circuit board.
 13. The protection deviceof claim 10, wherein the third terminal is bonded to a bond pad on thefirst side of the printed circuit board, the bond pad being electricallycoupled to a third conductive lead on the printed circuit board.
 14. Theprotection device of claim 10, wherein the variable transistor isconfigured to mount to a casing of a battery pack, and providesprotection of the battery pack from an overtemperature condition. 15.The protection device of claim 1, wherein the first terminal isconfigured for coupling to a positive terminal of the discharging loaddevice, the second terminal is configured for coupling to a positiveterminal of the chargeable element, and the third terminal is configuredfor coupling to a negative terminal of the discharging load device and anegative terminal of the chargeable element.
 16. The protection deviceof claim 1, further comprising a second variable resistor coupled inseries with the shunt regulator.
 17. A protection circuit for use with acharger and a chargeable element, comprising: a shunt regulator havingfirst and second terminals for coupling in parallel across thechargeable element, the shunt regulator having a threshold ON voltage;and a first variable resistor thermally and electrically coupled to theshunt regulator, the first variable resistor having a first terminal forcoupling to the charger in series and a second terminal for coupling tothe chargeable element in series, wherein the first variable resistorlimits current flowing through the shunt regulator during an overvoltageor overcurrent condition before the current reaches a level sufficientto cause the shunt regulator to fail, and wherein the shunt regulator isconfigured to mount on a first side of a printed circuit board, and thefirst variable resistor is configured to mount to a second side of theprinted circuit board.
 18. The protection circuit of claim 17, whereinthe first variable resistor switches from a relatively low resistance toa relatively high resistance when heated to a certain transitiontemperature, and wherein current flowing through the shunt regulator atthe predetermined level causes ohmic heat generation in the shuntregulator to cause the first variable resistor to substantially reachits transition temperature.
 19. The protection circuit of claim 17,wherein the first variable resistor comprises a positive temperaturecoefficient device.
 20. The protection circuit of claim 17, wherein theshunt regulator comprises a transistor switch.
 21. The protectioncircuit of claim 20, wherein the shunt regulator further comprisescontrol circuitry configured to activate the transistor switch if thevoltage across the chargeable element reaches the threshold ON voltage.22. The protection circuit of claim 21, wherein the control circuitrycomprises first and second voltage detection circuits, the first voltagedetection circuit being relatively low leakage and the second voltagedetection circuit being relatively precise, wherein the first voltagedetection circuit is configured to activate the second voltage detectioncircuit if the voltage across the chargeable element approaches thethreshold ON voltage, and wherein the second voltage detection circuitis configured to activate the transistor switch if the voltage acrossthe chargeable element reaches the threshold ON voltage.
 23. Theprotection circuit of claim 21, wherein the control circuitry comprisesan operational amplifier having an output coupled to an activation gateof the transistor switch, and further comprising a voltage clampingelement coupled to the operational amplifier output, the clampingelement thereby clamping the activation gate voltage.
 24. The protectioncircuit of claim 23, wherein the clamping element comprises a zenerdiode.
 25. The protection circuit of claim 20, wherein the transistorswitch has a thermally-compensated voltage characteristic.
 26. Theprotection circuit of claim 17, further comprising undervoltageprotection circuit.
 27. The protection circuit of claim 26, wherein theundervoltage protection circuit comprises a transistor switch having athreshold ON voltage, and having source and drain terminals configuredfor coupling in series between the charger and the chargeable element.28. The protection circuit of claim 27, wherein the undervoltageprotection circuit further comprises control circuitry configured toturn the transistor switch ON if the voltage across the chargeableelement is at or greater than a selected minimum voltage, and to turnthe transistor switch OFF if the voltage across the chargeable elementfalls below the selected minimum voltage.
 29. The protection circuit ofclaim 17, wherein the shunt regulator comprises a MOSFET switch having arelatively high resistance, reverse-current body diode.
 30. Theprotection circuit of claim 17, wherein the shunt regulator comprises atransistor switch having a channel through which current may flow in aforward direction if positive-biased, or a reverse direction ifnegative-biased, the switch being constructed such that current flowingin the forward direction encounters relatively low resistance, andcurrent flowing in the reverse direction encounters relatively highresistance.
 31. The protection circuit of claim 17, further comprising asecond variable resistor coupled in series with the shunt regulator.